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Distinguishing opinion from evidence in guidelines

BMJ 2019; 366 doi: (Published 19 July 2019) Cite this as: BMJ 2019;366:l4606
  1. Holger J Schünemann, professor12,
  2. Yuqing Zhang, assistant professor3,
  3. Andrew D Oxman, research director45
  4. on behalf of the Expert Evidence in Guidelines Group
    1. 1McMaster GRADE Centre and Department of Health Research Methods, Evidence, and Impact, McMaster University Health Sciences Centre, Hamilton, Ontario, Canada
    2. 2Department of Medicine, McMaster University Health Sciences Centre, Hamilton, Ontario, Canada
    3. 3Ningbo GRADE Centre, Nottingham Health China Centre, University of Nottingham Ningbo, China
    4. 4Centre for Informed Health Choices, Norwegian Institute of Public Health, Oslo, Norway
    5. 5University of Oslo, Oslo, Norway
    1. Correspondence to: H J Schünemann schuneh{at}

    The experience of experts can be useful when developing guidelines, but structures need to be in place to avoid opinion being confused with evidence, say Holger J Schünemann and colleagues

    Development of evidence based guidelines requires people with clinical, public health, or other relevant expertise and judgments about the evidence. The evidence underpinning those judgments should be identified, selected, appraised, synthesised, and presented systematically and transparently.123 But sometimes using evidence systematically and transparently can be challenging: evidence may be unpublished or indirect, diseases rare, contextual information missing, or resources limited. In these situations, obtaining evidence from experts can be efficient, and experts may be the only or main source of evidence.

    Using experts as a source of evidence has several problems and may seem at odds with evidence based medicine. However, its goal is to use expertise wisely in the context of evidence.4 We contend that there is a difference between evidence that comes from experts (expert evidence) and expert opinion, and argue that the way in which expert evidence is used in a guideline’s development has an important bearing on the robustness and trustworthiness of the guideline.

    How does expert evidence differ from expert opinion?

    Evidence in this context can be defined as facts (actual or asserted) intended for use in support of a conclusion.5 An opinion is a view or judgment formed about something, not necessarily based on facts.6 For example, a patient might say: “I had prostate cancer detected by prostate specific antigen (PSA) screening and I am alive 10 years later.” That is evidence. It is not the same as saying: “PSA screening saved my life.” That is an opinion. Similarly, a clinical expert might say: “I operated on 100 patients with prostate cancer and none of them died from prostate cancer.” That is evidence. It is not …

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